1. Field of the Invention
The present invention is related to the production of opto-electronic devices and more particularly to a method for producing an opto-electronic device by adjusting the resistance of load resistors after electrically connecting all components associated with the opto-electronic device.
2. Description of the Prior Art
The term "opto-electronics" refers to a general area of technology related to devices which emit, detect, or emit and detect electromagnetic radiation in the visible, infrared, or ultraviolet spectral regions of the frequency spectrum.
FIG. 8 illustrates an opto-electronic circuit used, for example, in an opto-mechanical mouse. The opto-electronic circuit includes a light emitting diode (LED) 810, a phototransistor 820, an encoder 830 located between the LED 810 and the phototransistor 820, a comparator 840 and a power supply 850. The anode of the LED 810 is connected to the power supply 850 and the cathode is connected to ground through an emitter load resistor 815. The collector of the phototransistor 820 is connected to the power supply 850 and the emitter is connected to the non-inverting input of the comparator 840. The emitter of the phototransistor 820 is also connected to ground through a detector load resistor 825. Finally, a reference signal is provided to the inverting input of phototransistor 840 through a voltage divider 860 formed by a first resistor 861 and a second resistor 862.
In operation, displacement of the opto-mechanical mouse on a flat surface results in rotation of the encoder 830. As the encoder 830 rotates, a slotted disk portion of the encoder 830 allows pulses of light to strike the base of the phototransistor 820. The light pulses cause the phototransistor 820 to turn on and off, thereby applying a series of high and low detector signals to the non-inverting input terminal of the comparator 840. Each time the amplitude of the detector signal rises above the reference signal, which is applied to the inverting input terminal, the comparator 840 generates a high OUTPUT signal which is transmitted to, for instance, a host computer (not shown) for controlling the position of a cursor on the viewing screen of a video terminal.
FIG. 9(a) illustrates an ideal detector signal 910, which is output from the phototransistor 820 (FIG. 8), and reference signal 920. The detector signal 910 is a sinusoid produced by a constant rotation of the encoder 830 (FIG. 8). The reference signal 920 has a constant amplitude which is midway between the peak amplitude levels of the ideal detector signal 910. FIG. 9(b) illustrates an ideal comparator output signal 930 generated in response to the ideal detector signal 910 and reference signal 920. As indicated, the ideal comparator output signal 930 has a duty cycle of 50%, thereby giving the host computer a maximum amount of time to detect the change of high or low output state. This permits the mouse to be operated at a faster speed than if the duty cycle were above or below 50%.
FIG. 9(a) also shows a non-ideal detector signal 950 having an average voltage which is shifted above the reference signal 920. This shift can be caused by, for example, the amount of light emitted by the LED 810, the sensitivity of the phototransistor 820, the resistance of the detector load resistor 825, the alignment of the LED 810 and phototransistor 820, and the position and quality of the encoder 830. For example, the light output from a single LED model can vary 10:1, and the sensitivity of a single phototransistor model can vary 4:1. In addition, resistors typically vary by 5% of their listed resistance. Any performance variations of these elements will cause the average voltage to shift up (or down) relative to the reference voltage, as indicated by the non-ideal detector signal 950. FIG. 9(c) shows a comparator output signal 960 which is generated in response to the non-ideal detector signal 950 and the reference signal 920. As indicated, the duty cycle of the comparator output signal 960 is substantially higher than the ideal 50% duty cycle of FIG. 9(b). This limits the operating speed of the mouse to a slower speed than if the duty cycle were 50%.
One prior art method attempts to produce mouse devices with ideal duty cycles by testing and matching compatible components prior to assembly onto a mouse device. The prior art method begins by "bench testing" each emitter and detector, and sorting the emitters and detectors by performance characteristics. For example, an emitter may be rated as a "low output" emitter and stored with other "low output" emitters in a bin. Likewise, "high output" emitters, "low sensitivity" detectors and "high sensitivity" detectors may also be stored together in their respective bins. Next, each emitter is matched with a detector having compatible characteristics. For example, an emitter from the "high output" bin is matched with a detector from the "low sensitivity" bin. Similarly, an emitter from a "low output" bin is matched with a detector from a "high sensitivity" bin. A detector load resistor is then selected to produce a desired detector signal level. The matched emitter and detector, along with a selected detector load resistor, are then connected to a production assembly, along with other electronic components associated with the mouse device. Finally, the encoder 830 is connected to the circuit board, along with other mechanisms associated with the mouse device, and the circuit board is mounted in a housing.
A problem with the above-described prior art method is that the matched emitter, detector and detector load resistor do not always produce the desired detector signal after they are assembled with other electronic and mechanical components associated with mouse devices. Because the emitter, detector and load resistor are electrically tested independently from the other components of the opto-electronic circuit, variations of the detector signal and reference signal caused by the other components cannot be accounted for, thereby resulting in detector signal shifts, as indicated by the non-ideal detector signal 950, which is shown in FIG. 9(a). Further, production variations of the encoders may cause shift from the ideal duty cycle shown in FIG. 9(b). For example, variations in the widths of the slots may effect the amount of light received by the phototransistor. Therefore, because the prior art method tests the detector signal level prior to complete assembly of the circuitry and the encoder, opto-electronic devices produced using the prior art method typically exhibit non-ideal duty cycles.
Another problem with the prior art method is that the process of sorting and matching emitters and detectors is laborious and expensive.